Home Forums Hunting & Shooting Ask The Gunwriters CHE as pressure indicator

Don't confuse the hardness of the base of a case with that of the neck. They should not be the same. FWIW, Dutch.

It's more elastic.

Lefteyed--

The .0004 expansion on Winchester .264 cases is what occured with factory loads in my rifle, so it should be safe--with that lot of brass. If the lot changes, everything changes.

Both the Norma brass and Winchester brass worked fine with max loads in my rifle. I have found Norma brass to vary considerbly in hardness from caliber to caliber. This .264 brass is certainly softer than Winchester, but their 9.3x62 brass is apparently pretty darn hard. Also, brass tends to work-harden as it's shot.

Have never tried to measure brass hardness, though no doubt it could be done. But I have grown so conservative in my loading over the years that I've never had a problem with too-soft brass.

This conservatism came from a lot of screw-ups, some on my part but the worst on others, and learning about pressure when i started to get serious as gun writer. Most of the time all you'll gain by pushing things is 100 or maybe 150 fps, which means nothing in the field. All you can lose is your rifle, your eyes or your life. If I want another 200 fps I use larger cartridge. The 7x57, for instance, kills just as well with a 140-grain bullet at 2800 as at 2950. If I want 3000 I'll use a .280.

When I can't read the headstamp after one firing, I sue the brass manufacturer for making soft brass! Isn't that what everybody does?

284lover--

I don't even trust traditional pressure signs much anymore, having found that in some kinds of brass they don't show up until you're over 70,000 psi anyway. Mostly I trust the chronograph anymore. If iy shows I'm getting 2900 out of a 140 in the 7x57, then I'm loading too hot!

MD

Thanks Mule Deer,

Yeah, maybe the .264 Win crowd could get a class action going against Norma... I mean they were all over you to work up some loads with all these newfangled superslow powders and now its the blasted brass thats going to hold you back <img src="/ubbthreads/images/graemlins/grin.gif" alt="" />.

I guess the bottom line is you really never can change ANY component, without dropping the charge and working back up.

One other thing. Based on the primer type vs pressure stuff you posted, do you ever recommend using magnum primers to work up loads in say, 30-06 volume cases?

284 -

ad 2 and 3: I fully agree

ad 1: Yes, there are "classic" pressure signs on the case - but they're not reliable

If you find classic signs, you may be high in pressure, but you don't know how high.
Sometimes classic signs show up without the load beeing in dangerous pressure levels.

I remember a proof house testing that ended up in a 100% pressure- and velocitywise perfect load, measured out of a proof barrel. This "otherwise fine load" (5,6x50 R) blew primers and showed enlarged primer pockets with every shot.
Everybody looked at the cases and said: "Where the hell these typical "classic" high pressure signs come from?"

In another incident a 7 mm Rem Mag blew the proof house pressure testing device (the piezo transducer to be correct): A highly recommended US- manual load was "copied" by a handloader. It shot with excellent accuracy, the cases showed absolutely no "classic" pressure signs (I have one in my collection). After having more than 200 rounds of this load fired at the range, the handloader asked a friend with access to a chronograph and they ended up measuring about 5% more velocity than predicted in the manual.
Because the handloader, a thinking, serious person, wanted to be sure, he went to the proofhouse. He brought some loads which he - "just to be more sure" - had been loaded up with about one half grain *more* powder (otherwise exactly the same components, same lot etc.).
This only "1/2 grain plus" load blew the instrument which means the pressure was way over gun pressure test level which is about 30% over CIP maximum. The guy was both happy (because unhurt, with a rifle still sound) and not very happy (he had to pay for the transducer unit)!

What else do we learn from this issue:

- Classic signs are, well, not worthless but not reliable

- "half a grain more" may blow your hand or head or both (or in this case someone elses "gun")

- what Mule Deer said: your chrono is kinda pressure device (and I have to add if you are lucky it will be your very very personal insurance)

Good luck
RD

JB, if velocity is a good guideline when working up loads, ie higher velocity equals higher pressure, how do you explain the phenomenon of Hodgdon Lil Gun in a 22 Hornet....higher velocity at lower pressure? As a Hornet shooter I enjoy the increased performance out of this little round but have marveled at this seeming anomaly.

Thanks for the reminder MD & RD. I seem to forget...

Li'l Gun appears to be the perfect powder for the Hornet. This happens now and then. In fact many of the advancemnets made in powder and factory ammo these days are related to the development of slower, denser ball powders.

When I talk of using a chronograph as a pressure gauge, I'm talking about looking at the top velocity for the powder you're using--and while using the same primer and bullet in the loading data. Trying to get Li'L gun velocities out of 2400, for instance, would be foolish.

Here is how I use a chronograph, for the most part. I look up loads and note the exact combination of powder, bullet and primer, say a 139 Hornady Spire Point, 50 grains of H4350, Remington case and Remington 9-1/2 primer in the Hodgdon manual. This gives a top velocity of 2906 fps with the max load.

This seems like it might work, but I don't have any Remington primers, and my barrel is only 22". So I sub CCI 200's (one of the other milder primers on the market) and allow 25 fps for each inch of barrel. This means I should be able to reach 2850 fps safely, but no more.

I shoot this combo, working up from Hodgdon's starter load of 47 grains. While 50 grains gives no traditional "pressure signs," velocity in my barrel at 15 feet is 2879, meaning about 2894. This is a little more than should be safe in a 22" barrel, judging from the Hodgdon data, so I drop back to 49.0 grains. Velocity is now under 2850, accuracy still good, the deer in just as much danger--and I'm not.

You are probably on safe ground subbing a CCI 200 for any magnum primer, but not the other way around. From all indications even the Winchester Large Rifle primer is pretty hot, so don't sub it for a CCI 200.

But above all, watch the chronograph. If you are getting 100 fps more with a certain load than the manuals suggest, then your load is probably too hot. I would rather err on the safe side myself, having found that 100 fps plus or minus doesn't make any difference in the field.

MD

...You are probably on safe ground subbing a CCI 200 for any magnum primer, but not the other way around....


Ja - I initiated a test with RWS laboratory and in some instance we got up to 500 bar piezo more pressure when switching from Standard to Magnum primers. This was in max level.
Otherwise some loads in 75-85 % max. (mostly mild loads in rimmed cases) could be improved in accuracy when switching from Standard to Magnum primers.
Regards
RD

The only advantage to soft brass over hard would be an ability to better seal the chamber at low pressures. This apart from the obvious benefits of a relatively soft neck and shoulder portion.
Another point which is often missed is that when case head expansion is being observed, it may be more a case of distortion than expansion. In other words, expansion of the case ahead of the web distorts the brass at the point of measurement. I suspect this is the reason some like to think a chamber which is tighter in diameter at the base will limit case head expansion. What it really does is limit distortion.
Now, this is not to say that a tight chamber is not a desirable trait (within reason) or that close tolerances have no effect on case head expansion (they can). What I'm saying is, in most cases the shooter is seeing a reduction in distortion rather than expansion. Keep in mind that in the case of a Remington 700, for instance, at least .150" of the cartridge is protruding from the chamber or unsupported. If pressure is great enough to cause expansion of the case head, it will do so regardless of chamber dimensions. This will occur when the pressure exceeds the elatic limit of the particular brass. That is to say, when the brass does not spring back to it's original size.
At this pressure level, a chamber which is very tight, in a rifle which encloses more of the case ( like a Ruger No1 for instance) will limit measurable case head expansion. This because, although the pressure was enough to exceeed the elastic limit of the brass, the brass did not expand to a larger dimension so the case head remains the same size. In this case there will be some changes in the brass however and the primer pocket may still enlarge. If the pressure is increased beyong this level the brass becomes more plastic and will eventually even liquify. Not a good situation!
Another point mentioned by Mule Deer is that the brass will harden from firing. This is because brass is hardened by working. So, even if the brass only expands a miniscule amount then springs back, it is still being worked and will harden. Because of this work hardening, brass which has expanded .0005" on the first firing with a given load may not expand any further when fired again with the same load or may expand slightly less.
It is also apparent that brass will harden and become brittle just sitting there. This is why very old brass may split at the neck or shoulder although it has not been worked at all.GD

Greydog: Amen to that. Case head expansion does serve as a warning, sorta like the idiot light on your dashboard.
Take Care!

I just purchased Hodgdon's 2004 Annual Manual. There are two articles by Rick Jamison dealing with this very issue. Final analysis seems to be learn to use as many "signs" as possible...don't rely on any one alone. Point well made by him and all who participated in this discussion. Thanks.

Jackfish:

Denton Bramwells article contains some "truths" some egregious errors and some misconceptions.

* One of the truths is that the case head measurement can't distinguish useful pressures between 30,000 and 70,000 psi.

Here are some vital considerations:

* There is no correlation at all between psi and cup. Sometimes, in some cartridge types, psi is more, sometimes it is the same and sometimes it is less than cup.

* Ammunition and artillery shells are made from one particular metallurgical type 280 brass. The ingredients are the same but the case thicknesses are not. This is particularly true about military brass compared to sporting brass.

* Different cartidge shapes, lengths, diameters, and their volumetric ratio to barrel volume *( caliber ) affect pressures in different ways.

* Using primers that are large rifle, large rifle magnum and vice versa in cartridges have a very pronounced effect on the amount of pressure. Changing from one manufacturer *( Winchester, CCI Remington, Federal ), and changing lot numbers can cause pressures to vary as much as 12,000 psi. Every time different primers, or lot numbers, or bullets, or powders, or grains of powder, or cases, or case lot numbers were changed a whole new set of calibrations would be necessary to correlate with case head expansion.

* Of the over 122 different types of gun powdwers available some are faster burning, some are slower burning, some have various shapes, ball, spherical, tubular, hollow tubular, flake, some use retardants, some are single base nitro-cellulose , some are double base *( impregnated with nitro glycerine ) none of these can be correlated with case head expansion and psi and standardized.

* Barrel lenght, internal volume and temperature will cause the same loadings to vary in pressure.

* Brass will variably continue to expand with each subsequent firing.

* Case head expansion will vary from one case to another.

* Belted magnum cases will sometimes enlarge so much, *( at times by the 3rd firing ) that they cannot be rechambered even with full length resizing because standard dies cannot resize near the belt.

* When cases are resized all previous case head expansion readings go out the window.

* The so called "pressure ring", the shiny ring that develops a short distance above the extractor groove or belt is actually an indication or the differential brass's flexing *( fulcrum point ) as it balloons, in and out, and this is due to the differences in the brass thickness in the cases base and the cases body. It is a slow tapered transition and the ring occurs at the thinnest portion which remains the same thickness up to the shoulder. Thus the "pressure ring" doesn't actually represent pressure, rather it representd brass working and if it did repersent pressure it would only be valid at that exact point and likewise would be non-representative of the brass/pressure in the base and in the body because of the variations in thickness. Therefore, if would be a practically useless number even if it could be interpreted.

* Brass cases will vary in their enlargement because they work harden with repeated reloading. Thus a first fired brass and a 5th or 6th fired may not, and usually will not, give the same case head expansion readings.

* Brass cases act as vessels which capture/contain some pressure isolating it from the barrels steel chamber. Thus the actual pressure acting on the steel chamber must be adjusted downward to compensate for the percentage absorbed by the brass. Since different brass has differen thickness, from cartidge to cartridge, and from th cases web to it's shoulder, the thicker brass near the base *( 4 to 5 times thicker than near the neck ), pressures read near the extraction groove will be subdued or reduced due to the thicker brass. The actual pressures in the thinner upper sections may be much higher than actually indicated by the micrometer readings.

* Strain gauges taped to the chamber cannot be correlated with case head expansion for the above reason. The strain gauge readings will, however, be more reliable because they are reading the steels expansion externally.

Bill Tibbe

There is indeed a correlation between psi and cup. Maybe you mean there is no direct correlation between them. Precision in language is probably as important as precision in measurement.

Bill--

Actually, one guy (a retired engineer) has worked out a psi/cup formula, and submitted it to RIFLE. But I believe it only worked with IMR-type single-base powders, which makes it fairly useless.

Have experimented with the "pressure ring" and have found that measuring it can come fairly close to predicting safe pressure if:

1) Brass is the same as that used in a factory round.
2) The ring is only measured on once-fired brass.
3) You also have "reference ammo" (normally a factory load, or a handload of known pressure) to compare it to.

Tested this against piezo readings and it comes out wihtin 3000 psi or so. No, this isn't exact--but it's the only way I've ever come to trust case measurements as any sort of pressure indicator.

MD

So, if it might have limited value for standard cartridges it appears it is completely worthless for wildcat cartridges requiring fireforming brass.

I would say so.

MD

MD,
Is this the author you refer to with the formula for PSI to CUP? He seems to get a good relationship with out reguard to powder. There is also a interesting view on CHE and PRE with the results of testing he`s done with them..